1. Field of the Invention
The present invention relates to a television tuner, particularly to a television tuner in which mutual interference is reduced between circuits to be mounted on a circuit board.
2. Description of the Related Art
In regard to a traditional television tuner, the layout of circuit components to be mounted on a circuit board is shown in FIG. 3. FIG. 3 is a plan view illustrating the inside of a television tuner, depicting a circuit board 32 mounted in a rectangular frame 31 and the layout of circuits disposed on the circuit board 32. The frame 31 is formed of a first side plate 31a bent in a horseshoe shape and a second side plate 31b for connecting two sides facing each other in the first side plate 31a in which the circuit board 32 is mounted in the rectangular area surrounded by the two side plates 31a and 31b. Then, a connector 33 for taking in television signals is mounted on the first side plate 31a, and a plurality of terminals 34 to be connected to the circuit board 32 is mounted on the second side plate 31b. 
The circuit board 32 is separated into three block areas 32a, 32b and 32c as shown in the drawing, and shield plates, not shown, are erected along the borders (indicated by dotted lines) between the block areas adjacent to each other. Then, an input tuning circuit (not shown) is formed in the first block area 32a disposed on the connector 33 side, and a high frequency amplifier circuit and an interstage tuning circuit (both not shown) are formed in the second block area 32b disposed inbetween. In addition, an intermediate frequency circuit for converting the inputted television signals to intermediate frequency signals and for detecting intermediate frequency signals is formed in the third block area 32c most separated from the connector 33.
The intermediate frequency circuit has the configuration as shown in FIG. 4, and a first oscillation signal is fed from a first oscillator circuit 42 to a first mixer circuit 41 for converting frequencies. The first oscillator circuit 42 is formed of a first oscillator transistor circuit 42a and a first resonant circuit 42b coupled thereto. Then, the first mixer circuit 41 and the first oscillator transistor circuit 42a are formed in a first integrated circuit component 40. An intermediate frequency tuning circuit 43 is connected to the subsequent stage of the first mixer circuit 41, and an intermediate frequency amplifier circuit 44 is connected to the subsequent stage of the intermediate frequency tuning circuit 43. The intermediate frequency amplifier circuit 44 is also formed in the first integrated circuit component 40. A SAW filter 45 is connected to the subsequent stage of the intermediate frequency amplifier circuit 44.
A second mixer circuit 47 for synchronous detection is connected to the subsequent stage of the SAW filter 45. A second oscillation signal is fed to the second mixer circuit 47 from a second oscillator circuit 48. The second oscillator circuit is formed of a second oscillator transistor circuit 48a and a second resonant circuit 48b connected thereto. Among them, the second oscillator transistor circuit 48a is formed in a second integrated circuit component 46 along with the second mixer circuit 47.
And, when a television signal to be received is inputted to the first mixer circuit 41, it is mixed with the first oscillation signal outputted from the first oscillator circuit 42 and converted to an intermediate frequency signal. The intermediate frequency signal is inputted to the second mixer circuit 47 through the intermediate frequency tuning circuit 43, the intermediate frequency amplifier circuit 44 and the SAW filter 45. Then, it is mixed with a second oscillation signal (the same frequency as a video intermediate frequency in an intermediate frequency band) outputted from the second oscillator circuit 48 and is synchronously detected to obtain a video signal.
Next, the detailed layout of the intermediate frequency circuit formed in the third block area 32c will be described. The third block area 32c has a nearly square shape, and the first integrated circuit component 40 and the second integrated circuit component 46 are disposed in small areas diagonally splitting the square area. Additionally, the first resonant circuit 42b is formed adjacent to the first integrated circuit component 40, and the second resonant circuit 48b is formed adjacent to the second integrated circuit component 46. Furthermore, the intermediate frequency tuning circuit 43 is formed in the area between the area formed with the first integrated circuit component 40 and the area formed with the second resonant circuit 48b, and the SAW filter 45 is formed in the area between the area formed with the second integrated circuit component 46 and the area formed with the first resonant circuit 42b. The first resonant circuit 42b, the second resonant circuit 48b and the intermediate frequency tuning circuit 44 are formed of components such as coils and condensers, and these components are mounted on the circuit board 32.
Power source voltage is fed to the first integrated circuit component 40 and the second integrated circuit component 46 by a power supply line 49 made of a conductor formed on the circuit board 32. A terminal 34a applied with the power source voltage is disposed at the position corresponding to the second block area 32b. On this account, a common power supply line 49a connected to a terminal 34a is disposed from the second block area 32b to the third block area 32c, and is connected to two power supply lines 49b and 49c in the third block area 32c. The power supply line 49b, one of them, is disposed between the area formed with the intermediate frequency tuning circuit 43 and the area formed with the second resonant circuit 48b, and the power supply line 49c, the other of them, is disposed in the area opposite to the power supply line 49b as the second resonant circuit 48b is sandwiched. Then, current is fed to the first integrated circuit component 40 through the power supply line 49b, one of them, and is fed to the second integrated circuit component 46 through the power supply line 49c, the other of them.